Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so safety is covered. The description adds genuine behavioral context beyond that: it is a subscriber tool, it preserves the requested view and method metadata, and it specifies the exact failure mode for unauthenticated callers.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences, no filler, with the core return scope front-loaded and access constraints placed last. Slightly more verbose than strictly necessary but every sentence carries information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return values need not be described. Combined with full schema coverage and annotations, the description supplies the missing auth/subscription layer, leaving no critical gap for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so every parameter is already documented in the schema; baseline is 3. The description confirms the three view values and the forward method concept but adds no syntax or defaulting detail beyond what the schema provides.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb (return) and resource (government-nominal curve nodes, slopes, derived forward segments) scoped to one currency, which is narrow and identifiable. It does not name a sibling to distinguish itself from close alternatives like rate_differentials or fx_reference_universe, so it stops short of a 5.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description communicates access conditions (API key required, anonymous calls return subscription_required with a subscribe link), which is useful gating context. However, it offers no when-to-use versus rate_differentials or fx_reference_universe, so usage selection is left implied.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.